The invention relates to aqueous well fluids containing an oxidized chitosan-based compound and to methods of drilling and servicing wells using such fluids.
As hydrocarbon-producing wells mature, water production becomes a serious problem. Remediation techniques for controlling water production are generally referred to as conformance control. Remediation techniques are selected on the basis of the water source and the method of entry into the wellbore. Conformance control treatments include sealant treatments and relative permeability modifiers (also referred to as disproportionate permeability modifiers).
In previous years, water-soluble chromium (+3) crosslinked polyacrylamide gels have been used in conformance control treatments. The gel time was usually controlled by the addition of materials that chelate with chromium in competition with the polymer-bound carboxylate groups. The crosslinking reactions in these gel systems take place by the complexation of Cr (+3) ions with carboxylate groups on the polymer chains. Because of the nature of the chemical bond between Cr (+3) and the pendant carboxylate groups, formation of insoluble chromium species can occur at high pH values. Other problems with these systems include thermal instability, unpredictable gel time, and gel instability in the presence of chemical species that are potential ligands.
Another water-based gel system for conformance control is based on phenol/formaldehyde crosslinker system for homo-, co-, and ter-polymer systems containing acrylamide. The crosslinking mechanism involves hydroxymethylation of the amide nitrogen, with the subsequent propagation of crosslinking by multiple alkylation on the phenolic ring. Because of the nature of this chemical bond, the gel time is controllable over a wide temperature range. Although these gels work well, phenol and formaldehyde are highly toxic.
U.S. Pat. No. 5,836,392 discloses a system based on a polyethyleneimine (PEI) crosslinker and a copolymer of acrylamide and t-butyl acrylate (PA-t-BA). PEI is such a low-toxicity material that it has been approved in the United States by the Food and Drug Administration for food contact. Although non-toxic, PEI may bio-accumulate or persist in the environment for long periods.
Recently, U.S. Pat. No. 6,291,404 and U.S. Pat. No. 6,258,755, disclose the use of chitosan as a non-toxic, biodegradable component for use in drilling fluids. Chitosan""s usefulness as a crosslinker has been limited, however, by its relative insolubility in aqueous solutions. For example, commercial sources of chitosan are only sparingly soluble in water; about 1% active solutions are the highest concentrations that can be made while maintaining usable viscosity. Therefore, chitosan has only been used at low concentrations as crosslinkers in conformance gels. While this is a step forward in the effort to provide more environmentally-acceptable systems, the major component of such a gel system is still a non-biodegradable polymer.
The present invention provides a well treatment fluid containing water, an oxidized chitosan-based compound, and a water-soluble compound having carbonyl groups. The oxidized chitosan-based compound has the property of being soluble in water at greater than about 2 wt % while maintaining a viscosity of less than about 1000 centipoise (cp). According to one embodiment, the water-soluble compound having carbonyl groups includes an acrylamide-based polymer. According to another embodiment, the water-soluble compound having carbonyl groups includes an oxidized starch. The present invention also provides a method of treating a subterranean formation penetrated by a wellbore comprising the steps of: (a) forming the well treatment fluid, and (b) contacting the subterranean formation with the fluid.
These and other embodiments of the present invention will be apparent to one skilled in the art upon reading the following detailed description. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof will be described in detail and shown by way of example. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the invention is to cover all modifications and alternatives falling within the spirit and scope of the invention as expressed in the appended claims. The compositions can comprise, consist essentially of, or consist of the stated materials. The method can comprise, consist essentially of, or consist of the stated steps with the stated materials.